1. Field of the Invention
The present invention relates to an antenna, and in particular to a multi-band antenna employed in a mobile electronic device.
2. Description of the Prior Art
In 1999, the wireless local area network (WLAN) market saw the introduction of the 2.4 GHz IEEE 802.11b standard. Today 802.11b and IEEE 802.11a are among several technologies competing for market leadership and dominance.
The wireless 802.11a standard for WLAN runs in the 5 GHz spectrum, from 5.15-5.825 GHz. 802.11a utilizes the 300 MHz of bandwidth in the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. Although the lower 200 MHz is physically contiguous, the Federal Communications Commission (FCC) has divided the total 300 MHz into three distinct 100 MHz realms; low (5.15-5.25 GHz), middle (5.25-5.35 GHz) and high (5.725-5.825 GHz), each with a different legal maximum power output in the U.S.
802.11a/b dual-mode WLAN products are becoming more prevalent up in the market, so there is a growing need for dual-band antennas for use in such products to adapt them for dual-mode operation. A dual-band planar inverted-F antenna (PIFA) is a good miniaturized built-in antenna for mobile electronic products. However, the bandwidth of the conventional dual-band PIFA antenna is not wide enough to cover the total bandwidth of 802.11a and 802.11b. Generally, because of this narrowband characteristic, the bandwidth of the dual-band PIFA can only cover the bandwidth of 802.11b and one or two bands of 802.11a.
One solution to the above problem is to combine two, or more than two, types of antennas. For example, U.S. Pat. No. 6,204,819 B1 discloses an antenna combining a PIFA and a loop antenna, which are selected by a plurality of switches. Though this antenna can achieve wider bandwidth by adjusting the parameters of the loop antenna, the tridimensional structure of this antenna occupies more space in an electronic device, and the employment of those switches increases the complexity and the cost of this antenna.
Hence, an improved antenna is desired to overcome the above-mentioned shortcomings of existing antennas.